To run eunit tests with rebar you can either create a Rebar Eunit run configuration or use a right-click context menu item 'Run ...' or 'Create ...' having clicked on a module(s), a unit test or a generator function you want to run.

Rebar-based Eunit test runner:

To set rebar as a default build action make sure you have checked Compile project with rebar checkbox:

After that your project will be built with 'rebar compile' command when you run Build > Make Project.
Compilation result:

Version 0.5 introduces experimental Erlang debugger support.
In order to use debugger you need to have debug_info checkbox under Settings > Compiler > Erlang Compiler checked.
You also need an Erlang Application run configuration which will be used by debugger. Such configuration can be created by right-clicking on entry point function.
Then you can use this configuration to debug your program:
The debugger requires Erlang Port Mapper Daemon (epmd) to be running. If it's not up, a warning will be issued.
To start the daemon run any Erlang node on your machine, for example with erl -sname foo -s init stop -noshell command.